The concept of using penetrating photons to examine the interior regions of objects that cannot be observed directly is about 100 years old. The revolutionary discovery of X-rays by Roentgen in 1895 led promptly to the development of non-destructive, non-invasive interrogation techniques applicable to various objects including the human body. Since the time of Roentgen, this method has developed enormously and now finds routine application in practically every aspect of modern life, e.g., manufacturing, construction, quality control, medicine, defense, transportation, security and basic and applied research.
The fundamental principles of photon radiography are well known and widely described in the literature. The most widely used approach involves X-rays in the range of a few keV to several hundred keV that are produced at relatively low cost by electron bombardment of medium to high atomic number metals in sealed, evacuated X-ray tubes. While this approach is extremely versatile, there are limits based on the penetrating capacity of these photons and on attainable source intensities. Photons with higher energies and source intensities can be obtained from radioactive gamma-ray sources, e.g., .sup.60 Co (or .sup.137 Cs) and from electron accelerators such as linacs and synchrotons. Radioactive sources are difficult to handle and store safely. Also, the range of geometric configurations that are possible with these materials is somewhat limited, mainly due to safety considerations. Accelerator sources are capable of producing very high radiation intensities and relatively high photon energies, but like X-ray tubes, they involve continuous energy photon spectra. These machines are also generally rather costly to build and operate. Because photon transmission through matter is highly energy dependent, radiography with continuous energy sources generally suffers from lack of adequate contrast and the inability to select proper exposure.
The present invention addresses the aforementioned limitations of the prior art by providing a radiographic method and apparatus which provides essentially monoenergetic, variable intensity, highly penetrating photons in an arrangement which is relatively inexpensive, safe and flexible in configuration for various applications.